In communications systems, a transmitter sends data streams to a receiver in symbols, such as bits of data. During data transmission, the data signal is usually distorted in terms of phase and amplitude due to various types of noise, such as fading, oscillator drift, frequency and phase offset, and receiver thermal noise. Signal degradation in high-speed data links can be mitigated by means of channel equalization. Continuous Time Linear Equalization is a common equalization technique to compensate the effects of a channel's transfer function (s-domain), particularly by flattening the frequency response within the frequency range of interest and removing Inter-Symbol Interference (ISI) caused by pre-cursors and post-cursors. As a transmission channel usually acts like a low-pass filter as it attenuates high frequency signal contents, a Continuous Time Linear Equalizer (CTLE) is designed to filter a received input signal by either boosting the high frequency content or attenuating the low frequency contents in the received signal.
A CTLE circuit can be implemented by using resistors, transistors, inductor, and/or capacitors. A typical CTLE transfer function exhibits one zero frequency and two pole frequencies as can be represented by a Bode Plot. The transfer function can be adjusted by varying the zero and/or pole frequency which can be achieved by changing electrical properties of the circuitry components in the CTLE. For example, the zero frequency location can be tuned by changing the voltage of a varactor. Placements of the zero and pole frequencies should be tailored to the channel characteristics to provide efficient channel equalization. Generally, when the zero frequency shifts to a lower frequency, the CTLE frequency response is augmented which corresponds to a higher gain for a given frequency in the interested band; and when the zero frequency shifts to a higher frequency, the CTLE frequency response is reduced which corresponds to a lower gain at a given frequency.
Conventionally, an optimal setting of a zero or pole frequency is obtained by scanning the numerous possible settings and selecting one that yields the best performance as evaluated based on Signal-Noise Ratio or Bit-Error Rate (BER) in the equalized signal. This trial-and-error approach takes a long time to converge and is not adaptive to channel changes over the time.